1. Field of the Invention
The present invention relates to a pressure seal for a coaxial cable connection utilizing F-Type connectors.
2. Discussion of the Related Art
Signal quality in systems employing coaxial cable is adversely affected when moisture from the environment enters the region bounded by coaxial cable end connectors. The vehicle transporting moisture from the environment into the cable is ambient air. Where this gas exchange is stopped, the transport of moisture into the cable is prevented.
The ingress of moisture into the coaxial cable is primarily due to the pressure changes in small air pockets disposed within the cable during ambient temperature changes. Variations in ambient temperature cause ambient air and the moisture it carries to be drawn into the coaxial cable. Both the moisture and the eventual corrosion of conductors inside the cable, especially the shield conductors, degrade signal quality. Where outdoor coaxial cable connections are concerned, it has become customary to seal F-Type connectors to the cable and to seal the interconnections between male and female connector parts.
The F-Type connector-to-coaxial cable, and F male-to-female connector interface have four places where moisture may enter the interconnection. The points of moisture entry are the interface between: (a) the trailing end of the male connector and the cable 60; (b) the connector shell and the connector body 61; (c) the swivel nut and the connector body 62; and (d) the swivel nut and the F-Type female connector on the device being connected 63. The foregoing principal sites of water vapor ingress are illustrated in FIG. 1.
Strong industry focus on cable to connector seals has resulted in several designs gaining acceptance in the industry as means for sealing the first three moisture ingress locations mentioned above.
However, no such industry focus on connector to connector pressure seals has occurred. And, to the extent that connector to connector pressure seal solutions have been developed, they are external seals. Moreover, no industry accepted design that effectively seals the last interface (i.e., the interface between the swivel nut on the male F-Type connector and the female F-Type connector on the device being connected) is available.
In accordance with the prior art, the leading end of the internally threaded nut on the male F-Type connector, which is attached to the cable, is screwed on to the female F-Type connector which has a mating outside thread. The integrity of the interface between the male and female F-Type connectors controls the mechanical and electrical performance of the connection. The thread used on F-Type connectors is a course ⅜-32 thread, specified by the SCTE (Society of Cable Television Engineers) and the EIA (Electronics Industry Association). This metal threaded interface does not provide an effective pressure seal for blocking gas exchange between the environment and the interior of the cable connection.
Known methods for preventing moisture ingress at connector to connector interfaces exist as shown in FIGS. 2a-e. All of these methods involve the use of external seals. In the case of the devices shown in FIGS. 2a-d, the illustrated device works only in particular applications. The device of FIG. 2e is somewhat more useful.
None of the prior art devices of FIGS. 2a-d provide an adequate seal between the nut on a male F-Type connector and the threaded shaft of a female F-Type connector which has threads on the exterior of the shaft. With reference to FIG. 2a, a rubber boot 10 is employed in accordance with the prior art to form a seal between a cable 11 and a ridge 12 that sometime exists on the female F-Type connector 13 mounted on the device 14 being connected to. The rubber boot 10 may keep out some moisture but does not provide a seal that is tight. Further, the device relies on the presence of a sealing ridge 12 on the female connector which is usually absent.
With reference to FIG. 2b, air shrink tubing 40 is also employed in the art to provide a seal between the cable 11 and the F-connector 13. Heat shrink tubing cannot be used because the PVC on the coaxial cable jacket will melt. The air shrink tubing 40 presents an inwardly-directed (radial) sealing force but requires a minimal length of the female F-Type connector shaft to be exposed in order to provide a water seal. In addition, the shaft must have a smooth surface. The tubing will not shrink into the threads of the female connector. Therefore this method has a limited application; being operable only for a female F-Type connector having a smooth, unthreaded outer surface on the shaft thereof.
Another sealing technique, though not widely used, is to fill the male connector nut with a silicone grease prior to attachment of the nut to the shaft of the female F-Type connector which will fill the area between threads. This is not recommended due to the difficulty in applying the correct amount of grease as well as the problem of removal and hand cleaning.
Yet another sealing technique, the axial compression port seal 20, is illustrated in FIGS. 2c and 2d. The axial compression port seal 20 consists of a tubular elastic member that slides over the shaft 21 of the female F-Type connector. When axial pressure from tightening the male nut 22 compresses the elastic device 20, the opposing end of the device exerts an equal force on a bulkhead 23 and thus seals both sides as it compresses. This device 20 and method works well if all sizes are exactly correct for the length of the shaft 21.
In practice, with many products being used, this method becomes ineffective. In addition, the axial compression port seal 20 relies on the axial force it exerts on a bulkhead in order to provide a seal. In many devices, this bulkhead does not exist. When an axial compression port seal 20 is used over threads, it cannot exert the needed inward radial force to fill and occlude the thread and pressure seal from its own elasticity. The radial sealing ability of axial compression port seals 20 has been limited due to the need for the installer to slide it over the cylindrical shaft of the female F-Type connector with little effort.
FIG. 2e shows the sealing design of U.S. Pat. No. 6,929,265 B2. Here, a compression ring 80 is advanced along an underlying elastic sealing member 90 when the abutting nut of a male connector is advanced along the threaded portion of a female connector 130. Seals made by this technique include the forward seal between the female connector and the elastic sealing member, the rear seal between the nut and the compression ring, and the seal between the elastic sealing member and the compression ring.
In summary, for the designs of FIGS. 2a-d, due to the variety of female connector port lengths, finishes, thread lengths, and the lack of clean, machined bulkheads for axial compression that are currently available on devices being used, it has been almost impossible to achieve moisture ingress protection. Even when a machined bulkhead is available for an axial compression seal, the seal must be sized for the exact length of the female port and male nut so that the proper axial force can be achieved when the male connector is fully screwed in. These three components may be sized correctly to resist moisture ingress for one set of products; but, the non-standardization of device dimensions used in the field make it highly improbable that each of these four variables (male nut depth; female shaft length; machined flat bulkhead; and axial rubber seal length) will be sized correctly in any particular installation. The design of FIG. e is an improvement, but it remains an external design that has not been widely adopted by the industry.